Simplification in method of measuring corrision of electronic conductors by non-gaseous ionic conductors



Dec. 18, 1962 3,069,332 LECTRONIC TORS R. G. SEYL SIMPLIFICATION INMETHOD OF MEASURING CORROSION OF E CONDUCTORS BY NON-GASEOUS IONICCONDUC Filed Oct. 22, 1959 Fig. 5

1H0 Fig.2

OCURRENT '0- 0 CURRENT Fig. 1

Inventor Robert G. 5e I by W Ci Attorneys United States Patent Office 3,[l69,332 Patented Dec. 18, 1962 3,069,332 SIMPLHFICATION IN METHOD (BFMEASURING CQRRISION OF ELECTRONIC CGNDUCTORS BY NON-GASEOUS IONICCONDUCTORS Robert G. Seyl, 1123 Mulford St., Evanston, 111. Filed Oct.22, 1959, Ser. No. 840,239 6 Claims. (Cl. 264-4) This invention isdirected to simplifications in the measurement of the corrosion currentdetermining the corrosion rate of an electronic conductor corroded by anon-gaseous ionic conductor, and is a continuation-in-part of myco-pending application Serial No. 778,211, filed December 4, 1958, nowabandoned, which is in turn a continuation-in-part of my co-pendingapplication Serial No. 659,459, filed May 15, 1957, now abandoned, whichis in turn a 'continuationain-part of my application Serial No. 282,935,filed April 18, 1952, now abandoned, which is in turn acontinuation-impart of my application Serial No. 786,499, filed November17, 1947, and now abandoned.

EFINITIONS Some terminology used in the art requires more specificdefinition when applied to this invention, and these definitions follow.

An electronic conductor conducts DC. current by electron flow. Elementalmetals and their alloys typify electronic conductors, but the classincludes substances which do not have metallic properties, as carbon andgraphite, and certain chemical compounds as oxides and sulfides.

A non-gaseous ionic conductor consists of an electrolyte dissolved in anionizing solvent, and conducts DC. current by the flow of positive ionsin one direction and the fiow of negative ions in the oppositedirection. The ionic conductor hereinafter referred to excludes thegaseous type of ionic conductor also known to the art.

A corrosion interface is here defined as that boundary region betweenthe surface of an electronic conductor and an ionic conductor in contacttherewith, within which occur the electrochemical corrosion reactions ofion formation and discharge produced by electric current, and withinwhich these corrosion reactions may be affected by films formed on theelectronic conductor surface by physical adsorption, electrochemicalmigration, chemical combination, mechanical application, and othermeans.

The voltage existing across the interface bounding electronic andnon-gaseous ionic conduction is not directly measurable. It isindirectly measured as the voltage difference between said electronicconductor and the electronic conductor of a reference electrode inelectrical contact with the ionic conductor, and is termed the electrodepotential, with the nature of the reference electrode also identified.

Free electrode potential is here defined as the electrode potentialexisting when the interface bounding electronic and non-gaseous ionicconduction is free from voltage disturbances produced by or momentarilyresulting from externally produced current passed through the interface.

When a DC. current is passed through the interface bounding electronicand non-gaseous ionic conduction, the voltage across the interfacebecomes altered in value and a polarized electrode potential results.Polarization voltage is here defined as the difference between thepolarized electrode potential and the free electrode potential.

New terminologies essential to describing novel details of thisinvention are set out through the use of word capitalizations in thespecification and claims which follow. bilities of these interfaceelectrodes described and meas- OBJECTS The principal object of thisinvention is the provision of a simplified method for measuring thecorrosion rate of an electronic conductor surface corroded by anongaseous ionic conductor.

Another object of this invention is the provision of a method of theforegoing character which simplifies measurement of the corrosioncurrent of an electrochemical mechanism measured and described in detailin the preceding division of this invention.

Another object of this invention is a method of the foregoing characterwhich measures with practically instantaneous speed the corrosioncurrent that determines the corrosion rate.

A further object of this invention is a method of the foregoingcharacter which continuously measures this corrosion current.

THE ART This invention is directed to a simplified method for measuringthe corrosion current of the interface electrode system.

In my prior method, greatest simplification in measurement of thecorrosion current operating within a corrosion interface corroding atthe free electrode potential requires measurement by the method of theinvention of a range of current-potential relationship substantiallyincluding the free electrode potential of the corrosion interface andextending beyond the transition point occurring at minimum polarizationvoltage, and application of resolving operations to this measured rangeof current-potential relationship through previously measuredcharacteristic values of the voltage separation between consecutivetransition points and of the line slope voltage, to produce measurementof the component current-potential relationships of the interfaceelectrode system.

In the method described herein, the requirement for measuring initialrange of current-potential relationship is simplified to the requirementfor a single measurement of value of polarizing DC current and resultingpolarization voltage. The requirement for resolving operations issimplified to measurement of the corrosion current throughcharacteristic proportionality of the interface electrode system appliedto measured value of polarizing DC. current and resulting polarizationvoltage.

Distinctive advantages are produced through this simpli fied method ofcorrosion current measurement. The measured value of polarizing DC.current may be less than the value of the corrosion current beingmeasured, to minimize disturbance of the corrosion interface properties.The corrosion current may be measured within the substantiallyinstantaneous time lapse of about two minutes. The corrosion current maybe continuously measured through the time interval within which thepolarizing DC. current does not unduly alter properties of the corrosioninterface. The method may be operated through a measurement device ofsmall size and weight, made simple to operate, with the meter measuringthe polarizing DC. current being operated to directly indicate the valueof the corrosion current being measured. The method may be operatedthrough a measurement device producing continuous recording of variationin corrosion current with passage of time.

PRELIMINARY DESCRIPTION The method described herein is based upon anelectrode system in which measurement is made of an interface electrodesystem characterized by interface electrodes having free electrodepotentials of 0.02:0.00'2 volt separation and by inter-related anodicand cathodic polaram3 ured by a line slope voltage of 0.02:0.002 volt,and found to operate through these characteristics without restrictionto interface composition and conditions of operation. This interfaceelectrode system defines an additional characteristic property of thecorrosion interface, which is here termed the direct voltage, and whichis equal to the sum of the anodic and cathodic polarization voltagesproduced on a corrosion interface by a value of anodic and cathodicpolarizing DC. current equal to the value of the corrosion current ofthe interface electrode system when the corrosion interface corrodes atits free electrode potential. This direct voltage occur within theinitial range of practically linear relationship between value ofpolarizing DC. current passed through the corrosion interface and valueof resulting polarization voltage produced across the interface. Asingle measurement of polarizing DC. current and resulting polarizedelectrode potential made within this range, with polarization voltagemeasured from additional measurement of the free electrode potential ofthe corrosion interface, produces measurement of the corrosion currentthrough this linearity of current-potential relationship when related tothe characteristic value of the direct voltage. This measurement method,here termed the simplified method, may be operated through a selectionof alternative electrode systems adaptable to laboratory and plantequipment handling corrosives.

This simplified method may be operated through the combination of acorrosion interface to be measured, a separate opposed and unmeasuredinterface, and a separate reference electrode.

Another combination includes a corrosion interface to be measured with aseparate opposed and unmeasured interface which remains unpolarized andoperates as the reference electrode.

Another combination includes a corrosion interface to be measured with aseparate opposed interface duplicating electrochemically the interfaceto be measured and of area no smaller than said interface, whichoperates as a reference electrode and which may contribute to themeasured resulting polarization voltage.

This simplified method may measure the corrosion current withoutsacrifice of accuracy in corrosion rate measurement. Maximum accuracy isobtained from the average of two consecutive measurements made withreversed polarity of the applied DC voltage.

THE FIGURES The following figures originate with the preceding divisionof this invention, and are repeated here to describe the measuringequipment and the principles upon which the simplified method operates.

FEGURE 1 is a diagrammatic section illustrating essential components ofmeasuring apparatus;

FIGURE 2 is a graph showing variations in shape of initialcurrent-potential relationship range when measured by my prior method.

FXGURE 3 shows undistorted ranges of anodic and cathodiccurrent-potential relationship measured by the method described in myprior method, and shows the corrosion mechanism of the interfaceelectrode system obtained therefrom by operations of resolving alsodescribed in my prior method.

BASIS OF SIMPLIFIED METHOD OPERATION The value of the direct voltageupon which operation of the simplified method relies is determined asfollows. Referring to FIGURE 3, point 37 measures a 40 unit corrosioncurrent operating at free electrode potential E of the corrosioninterface on which undistorted currentpotential relationship ranges E Cand E A were measured. In FIGURE 2 a current line is extended from this40 unit value of current to a point 38. The voltage intercepted alongthis current line by undistorted currentpotential relationships E C andE A is equal to the sum of the anodic and cathodic polarization voltagesproduced by a polarizing DC current of 40 current units, and is shown tobe 0.029 volt. This is a characteristic voltage, because thecurrent-potential relationships of FIGURES 2 and 3 are independent ofgraphical dimension of voltage unit and of current unit, and areindependent of the ampere value of the current unit. The characteristic0.0210002 volt separation between free electrode potentials of interfaceelectrodes and the characteristic 0.02:0.002 volt value of line slopevoltage of the interface electrode system determine the value of thedirect voltage independently of specific value of current unit. The 40unit current distance on these graphs may represent 4, 40, 400, or anyother microampere value of current measured on a specific corrosioninterface of specific area. This direct voltage may also be defined fromthe extension of initial lines B 17 and E -IS of undistortedcurrent-potential relationships E,C and E -A, which intercept 0.031 voltalong the current line at 40 current units.

Another principle upon which the simplified method operates is that onlya small amount of distortion of current-potential relationship may occurwithin a corrosio-n interface of unspecified composition and operationwhen polarized by a value of DC. current equal to the value of thecorrosion current of the interface electrode system operating at thefree electrode potential of the corrosion interface. Reference is madeto FIGURE 2 which shows various shapes of distorted current-potentialrelationships compared with undistorted relationships of E C and E -A.The distorted relationships are shown in the position of coincidingtransition point potentials and free electrode potential, and withcurrent units graphically proportioned so that the corrosion currentoperating at the free electrode potential of the measured corrosioninterface coincides with the 40 unit current value defined in FIGURE 3by point 37. Undistorted cathodic current-potential relationship E C anddistorted currentpotential relationship E -C intersect the 40 unitcurrent line at substantially the same potential. Undistorted anodiccurrent-potential relationship E A and distorted relationship E -A' alsointersect the 40 unit current line at substantially the same potential.Some forms of distortion, illustrated by current-potential relationshipE C may occur to a small extent within the initial range of thepolarizing DC. current. Distorted relationship Er-C and relationship E Aintercept 0.035 volt along the 40 unit current line.

The direct voltage may consequently range from about 0.029 to 0.035volt, and for actual measurement purposes the average value of0.032i0003 volt is taken. One way of applying this direct voltage is topolarize two corrosion interfaces of electrochemically duplicatedinterface properties and of equal areas by that value of D.C. currentproducing anodic and cathodic polarization voltages of sum equal to thedirect voltage. The value of this polarizing DC. current is then equalto the value of the corrosion current operating on the corrosioninterface when it corrodes at its free electrode potential.

Another principle upon which the simplified method operates is theobservation that the direct voltage is defined from approximately equalvalues of cathodic polarization voltage and anodic polarization voltage.This is be regarded to illustrated in FIGURE 2 by the showing of 0.015volt of anodic polarization, and cathodic polarization ranging from0.015 to 0.021 volt. The corrosion current occurring at the freeelectrode potential of a corrosion interface may accordingly be measuredas equal to that value of polarizing DC. current producing an anodic orcathodic polarization voltage equal to one-half of the direct voltage.

A further principle upon which the simplified method operates is thatthe relationship between polarizing DC. current passed through thecorrosion interface and the resulting polarization voltage producedWithin the range extending from zero to about 0.03 volt may be regardedas practically linear for present purposes. Referring to FIGURE 2, theextent of line slope change occurring at transition points 17 and 17 ofmeasured cathodic current-potential relationships is relatively small.The same is true of line slope change occurring at transition point 18of measured anodic current-potential relationship. It is not tooinaccurate to view current-potential relationship between points B and19, E and 19', B and 19", and between points E and 20 as being linearfor present purposes. Proportionalities of triangular relationship maythen be applied to measure values of polarizing D.C. current andresulting polarization voltage ranging up to about 0.03 volt, asfollows. The value of the corrosion current operating when the corrosioninterface corrodes at its free electrode potential is related to themeasured value of anodic or cathodic polarizing DC. current passedthrough the corrosion interface in the proportion that one-half of thedirect voltage is related to the measured value of the resultingpolarization voltage produced by the polarizing DC. current.

CURRENT-POTENTIAL RELATIONSHIP MEASUREMENT The simplified method may beoperated within a wide range of precision, from qualitative measurementto precision or quantitative measurement. Little additional effort isrequired to obtain high precision of measurement, and the details ofproducing such measurement are described below with the obviousunderstanding that principles employed may be applied in varying degree.

The corrosion interface to be measured is formed by contacting a singlesurface of electronic conductor 1 with ionic conductor 2. The separateand opposed interface required for passing the polarizing DC. currentthrough the interface to be measured is formed by contacting a surfaceof electronic conductor 3 with ionic conductor 2.

The area of the interface to be measured and the area of the separateand opposed interface are formed with a combination of regularity inshape, size, and opposed position selected to produce small range ofvariation in value of polarized electrode potential subsequentlyproduced on the interface to be measured by the polarizing DC. currentpassed between the two opposed interfaces. The accuracy of corrosioncurrent measurement depends upon the extent to which the measured valueof polarized electrode potential is indicative of the potential of theentire interface area. When localized corrosion occurs within theinterface to be measured, the localized areas of potential dissimilarityare short-circuited through paths of ionic conduction producing small IRvoltage drops measurable as a small range of variation in the freeelectrode potential, and the polarized electrode potential will occurwith corresponding small range of variation. The uniformity in formingthe paths of ionic conduction passing the polarizing DC. current betweenthe two opposed interfaces is limited to maintaining range of variationof voltage delivered to the measured interface within range of variationoccurring to the free electrode potential.

FIGURE 1 illustrates one combination in which the measured interface andthe separate and opposed interface are formed from rod electrodes ofidentical dimensions, with rod diameter made small compared to rodlength and with major axes placed in parallel relationship. Rod diameteris diminished with decrease of ionic conductor conductivity, and mayrange from about one centimeter with ionic conductors of goodconductivity to about two millimeters with ionic conductors of smallconductivity approaching that of distilled water. The interface area isproduced primarily through selection of rod length.

The reference electrode required for electrode potential measurement isestablished within the ionic conductor at a separation distance from themeasured interface sufficient to include total polarization voltageproduced on the measured interface by the polarizing DC. current.

A separation distance of no less than about Mr inch has provensatisfactory, and is regarded to include mechanical and concentrationpolarizations. The separation distance also tends to produce an averagemeasurement of the range of variation within which the electrodepotential may occur.

The reference electrode is also positioned to substantially excludevoltage produced by ionic conductor resistance to the conduction of thepolarizing DC. current. This may be accomplished by separating thereference electrode from the measured interface by a. distance nogreater than about /1 inch with ionic conductors of medium conductivity.

The corrosion current may be measured at any instant of time selectedafter electronic conductors 1 and 3 are placed in contact with ionicconductor 2.

The free electrode potential E of the corrosion interface generallyvaries with time, and small variation in potential E may represent alarge portion of the comparatively small polarization voltage to bemeasured. When accurate corrosion measurement is required, a firstmeasurement is made of the potential E in the form of the voltagedifference between the electronic conductor of the interface to bemeasured and the electronic conductor of the reference electrode.

A DC. voltage of selected polarity is then applied to electronicconductors 1 and 3 through a voltage delivery system selected from aclass which promptly produces approached equilibrium in the form of slowrate of change of the current-potential relationship, to polarize theinterface to be measured by a voltage within the range from slightlyabove zero to about 0.03 volt at the time of measurement. Two classes ofvoltage delivery systems meeting this requirement are broadlyillustrated in FIGURE 1 by battery 12, battery switch 13, potentiometer14 and polarity reversing switch 16 as follows.

One class of voltage delivery system promptly producing approachedequilibrium of current-potential relationship delivers the DC. voltagewith a definite extent of voltage regulation. Referring to FIGURE 1,when potentiometer 14 is of low resistance, the DC. voltage initiallyapplied to the electrodes from a selected potentiometer arm positionremains substantially constant, and the current-potential relationshippromptly approaches equilibrium in the form of decreasing rate of changeof decreasing polarizing DC. current. When potentiometer 14 is ofintermediate resistance, the DC. voltage initially applied to theelectrodes increases and the polarizing DC. current decreases, and thecurrent-potential relationship promptly approaches equilibrium in theform of decreasing rate of change of increasing polarization voltage anddecreasing rate of change of decreasing polarizing DC. current. Whenpotentiometer 14 is of high resistance, the DC. voltage initiallyapplied to the electrodes increases from a small value while thepolarizing DC. current may undergo little change, and thecurrentpotential relationship promptly approaches: equilibrium in theform of decreasing rate of change of increasing polarization voltage. Adefinite extent of voltage regulation may alternatively be produced bymanual adjustments made to the potentiometer arm after application ofthe DC. voltage, as through adjustments made to maintain the appliedvoltage constant, or as through adjustments made to maintain thepolarizing DC. current constant.

Another class of DC. voltage delivery system produces prompt approach ofthe current-potential relationship toward equilibrium by continuouslydelivering the DC. voltage at a substantially constant rate of changenot exceeding the selected slow rate of change at which measurement ismade of the polarizing DC). current and the resulting polarizedelectrode potential. The arm of potentiometer 14 is driven at a selectedconstant speed to deliver the DC. voltage at the selected rate ofchange.

A voltage delivery system may be selected from either of these classesdescribed above, without altering the accuracy of the corrosion currentmeasurement. The class continuously delivering the DC. voltage atconstant rate of change offers no advantage to the simplified methoddescribed herein, and has the disadvantage of limiting the time formaking the measurements to the time within which the resultingpolarization voltage remains Within the range for measurement by thesimplified method.

Measurement may be made of the value of polarizing DC. current andresulting polarized electrode potential as soon as the current-potentialrelationship initially approaches a rate of change slightly greater thanthat produced by rate of change of the corrosion rate. The simplifiedmethod may be operated at such low density of polarizing DC. currentthat rate of current-potential relationship distortion by the DC.current generally becomes negligible. Rate of change of the corrosionrate may vary from small value With fixed ionic conductor compositionand fixed corrosive environment, to larger values when measurement ismade during changing ionic conductor composition or changing corrosiveenvironment.

With a voltage delivery system of the class delivering the D.C. voltagewith definite extent of voltage regulation, a convenient practice is toobserve rate of currentpotential relationship change during 30 secondintervals through readings of DC. current or polarized electrodepotential, whichever is made most indicative by the extent of thevoltage regulation. Measurement may be made of value of polarizing D.C.current and resulting polarized electrode potential as soon as thecurrent-potential relationship has approached a constant rate of change.This constant rate of change may range from about 0.2% to 2% during the30 second time interval, depending upon rate of change occurring to thecorrosion rate.

With a voltage delivery system of the class continuously delivering theDC. voltage at a constant rate of change, the constant rate of change involtage delivery may range from about 0.015 volt per minute to slowerrates delivered to electrodes 1 and 3 when of comparable polarability,and measurement is made of the value of polarizing DC. current andresulting polarized electrode potential at any selected instant of timewhen the polarization voltage is within the measurement range of thesimplified method.

With a voltage delivery system of the class delivering the DC. voltagewith definite extent of voltage regulation, measurement may be made ofvalue of polarizing DC. current and resulting polarized electrodepotential at instants of time or continuously throughout an interval oftime after initial attainment of the approached equilibrium during whichchange of interface composition and operation do not reduce accuracy ofthe corrosion current measurement below a desired limit. The length ofthis time interval through which measurement may be continued dependsupon specific conditions of corrosion interface composition andoperation, and on the electrode combinations chosen in the measurementmethod. Progress of the corrosion, particularly when attended by changein ionic conductor composition or corrosive environment, may producesignificant change in free electrode potential when measured from aseparate reference electrode 4 or an unpolarizable electrode 3 ofcomposition different from electronic conductor it, but may producelittle if any disturbance in measurement accuracy when electrodes 1 and3 are of identical electronic conductor composition and electrode 3operates as the reference electrode. The continued passage of thepolarizing DC. current may produce many effects related to specificcorrosion interface composition, such as the formation or breakdown of aprotective film, and alteration of composition of ionic conductor layercontacting the ionic conductor surface. The length of this time intervalthrough which measurement may be continued may in general be determinedby removal of the applied DC. voltage, by allowing the corrosion tocontinue for a small time interval to restore its undisturbedperformance, and then by making an instantaneous measurement of thecorrosion current by the simplified method. If this instantaneousmeasurement continues the trend of corrosion current defined inpreceding measurement, it is reasonable to conclude that the accuracy inpreceding measurement was not being unduly impaired.

Upon the completion of intended measurements, the applied DC. voltage isgenerally removed to permit the corrosion interface to continue thecorrosion at its free electrode potential.

The value of the polarization voltage is calculated from the freeelectrode potential and the polarized electrode potential according todetails known to the art.

ALTERNATIVE ENTERFACE COMBINATIONS FOR MEASUREMENT The simplified methodmay be operated through various combinations applied to forming thecorrosion interface to be measured, the separate and opposed interfacefor passing the polarizing DC. current, and the reference electrodeinterface for electrode potential measurement, as follows.

In one combination, the corrosion interface area to be measured isformed with electronic conductor 1, the separate and opposed interfacefor passing the polarizing DC. current is formed with electronicconductor 3, and the reference electrode interface for electrodepotential measurement is formed with separate electronic condoctor 4.This combination is advantageous with ionic conductors of smallconductivity, through introducing negligible IR voltage from ionic DCcurrent conduction into measured value of resulting polarized electrodepotential by positioning electronic conductor 4 to locate electronicconductor 1 between electronic conductors 3 and 4, as is shown in FIGURE1.

Electronic conductor 3 may be of composition other than that ofelectronic conductor l, and then should not alter the properties of thecorrosion interface to be measured. Corrosion products of electronicconductor 3 should not enter into replacement reaction with thesubstance of electronic conductor 1, or form a protective film withinthe electronic conductor 1 interface. The possibility of hydrogen oroxygen liberation within the electronic conductor 3 interface shouldnot. alter the corrosivity of ionic conductor 2 much more than thatproduced by the electronic conductor 1 interface.

Electronic conductor 4 may be in direct contact with ionic conductor 2,and then should not alter the properties of the corrosion interface tobe measured. Alternatively, electronic conductor may contact ionicconductor 2 through a reference electrode ionic conductor, and thenmeans should be employed to prevent contamination of ionic conductor 2such as jelling the reference electrode ionic conductor in a capillarytip.

Measurement is made With this combination of interfaces as follows. Thefree electrode potential is measured between electronic conductors l and4-. Voltmeter 9 is of a class requiring negligible actuation current.Switch 11 connects the voltmeter across electronic conductors l and 4.The reversing switch may be. required if voltmeter S9 measures DC.voltage of single polarity.

The DC. current passed between electronic conductors 1 and 3 maypolarize the electronic conductor ll interface within the range fromslightly above zero to about 0.03 volt. Value of polarizing DC. currentis measured by meter lid. The resulting polarized electrode potential ismeasured between electronic conductors ii and 4. The corrosion currentoperating when the electronic conductor 1 interface corrodes at its freeelectrode potential is then'related to the measured value of polarizingD.C. current in the ratio that /2 of the direct voltage is related tothe polarization voltage calculated from the measured values of theresulting polarized electrode potential and the free electrodepotential.

In another combination, the corrosion interface area to be measured isformed with electronic conductor 1, the separate and opposed interfacefor passing the polarizing DC. current is formed with electronicconductor 3, and the reference electrode interface for electrodepotential measurement is the electronic conductor 3 interface, in whichthis interface is made of composition and operation to substantiallyresist polarization by measured value of polarizing DC. current, and inwhich the separation distance between the electronic conductor 1interface and the electronic conductor 3 interface is limited to thatintroducing negligible IR voltage from ionic DC current conduction intomeasured value of resulting polarized electrode potential. Thiscombination is advantageous when measurement is made in industrialequipment having containing walls of metal, such as pipes and tanks.

The interface formed by electronic conductor 3 may be made substantiallyunpolarizable by the combined factors of interface area and interfacecorrosion rate. When electronic conductor 3 corrodes at a ratecomparable to electronic conductor 1, the polarization of the electronicconductor 3 interface may be held to about one tenth that of theelectronic conductor 1 interface by forming the electronic conductor 3interface area of size ten times greater than the electronic conductor 1interface. Electronic conductor 1 may be in the form of rod positionedalong the major axis of a pipe, with the inside pipe surface forming theelectronic conductor 3 interface. The electronic conductor 3 interfacemay be formed by a tank wall near which electronic conductor 1 ispositioned. The effective interface area formed by an electronicconductor 3 surface is generally limited by IR voltage loss through theionic conductor to about ten times that of the electronic conductor 1interface area. This invention shows that initial range of interfacepolarability is inversely related to corrosion rate, and polarizationresistance may additionally be achieved through an electronic conductor3 interface of larger corrosition rate than that of the electronicconductor 1 interface. Alternatively, electronic conductor 3 may form areversible unpolarizable interface with a second ionic conductor, withthis second ionic conductor contacting ionic conductor 2 through anon-contaminating junction.

Electrode potential measurements are made with this combination asfollows. Voltmeter 9 is connected across electronic conductors 1 and 3through switch 11. The free electrode potential and the polarizedelectrode potential of the electronic conductor 1 interface are measuredbetween electronic conductors 1 and 3. The corrosion current operatingwhen the electronic conductor 1 interface corrodes at its free electrodepotential is obtained through the same relationship as that describedfor the preceding combination of measured interfaces.

In still another combination, the corrosion interface area to bemeasured is formed of area A with electronic conductor 1, the separateand opposed interface area for passing the polarizing DC. current isformed with electronic conductor 3 electrochemically duplicating theelectronic conductor 1 interface in the form of an area A of size nosmaller than A and the reference electrode interface for electrodepotential measurement is the electronic conductor 3 interface in whichthe separation distance between the electronic conductor 1 interface andthe electronic conductor 3 interface is limited to that introducingnegligible IR voltage from ionic DC current conduction into measuredvalue of resulting polarized electrode potential. This combinationoffers a convenient selection of electronic conductor material andproduces high accuracy of measuurement when area A is made equal to areaA to cause the simplified method to pro duce a measurement averaging theperformances of duplicated corrosion interfaces.

The free electrode potential of the corrosion interface is measured asthe potential difference between electronic conductors 1 and 3.Ordinarily this potential difference may be negligible, but in thepresence of unequally distributed localized corrosion occurring withinthe duplicated interfaces it may exceed 0.02 volt. When the freeelectrode potential does not exceed the applied voltage, its effecttends to be cancelled out with two consecutive measurements made by thesimplified method with reversed polarity of the applied DC. voltage,through the averaging of the corrosion current values obtained from thetwo measurements.

The principles of simplified method operation remain unaltered whenmeasurement is made with these duplicated interfaces of areas A and Aand the details consist of handling a polarization voltage of onepolarity with a polarization voltage of reversed polarity in an additivemanner which is measured between electronic conductors 1 and 3, asfollows. The same value of measured polarizing DC. current passesthrough each of the duplicated interfaces, polarizing one anodically andthe other cathodically. The polarizing DC. current density passingthrough the interface of area A is therefore less than that passingthrough the interface of area A by the factor, A /A Since the simplifiedmethod is operated within the substantially linear initial range ofcurrent-potential relationship, the polarization voltage produced acrossthe interface of area A is less than that produced across the interfaceof area A by this factor, A /A Consequently when the interface of area Ais polarized within a range of voltage up to about 0.03 volt, theinterface of area A is polarized by a voltage up to (A /A (0.03) volt,so that the total polarization voltage calculated from the freeelectrode potential and the polarized electrode potential, each measuredbetween electronic conductors 1 and 3, may range up to (l-l-A /A (0.03)vol-t. In calculating the corrosion current, the proportionality term(l/2)(direct voltage) applies to the single polarity of the polarizationvoltage produced on the interface of the area A Since the polarizationvoltage produced across the interface of area A is included in thepolarization voltage measurement, its effect must be included byincreasing this proportionality term through addition of the quantity (A/A )(l/2) (direct voltage), so that the total proportionality termbecomes (l/2)(1+A /A )(direct voltage). The corrosion current operatingwhen the electronic conductor 1 interface of area A corrodes at its freeelectrode potential is then related to the measured value of thepolarizing DC. current in the ratio that the term (1/2)(1+A /A )(directvoltage) is related to the polarization voltage calculated from themeasured values of the resulting polarized electrode potential and thefree electrode potential, and electrochemically duplicated interfacearea A may contribute to the precision of the measurement. Thus, whenarea A is of such large size that it is practically unpolarized, thedirect voltage proportionality term becomes (1/2) (direct voltage); andwhen area A is made equal to area A the direct voltage proportionalityterm becomes (direct voltage), and the corrosion current is measuredwith the increased precision obtained from the averaged response of twoduplicated and measured interfaces.

RATE MEASUREMENT BY CORROSION CURRENT The accuracy with which thissimplified method may measure the interface electrode system corrosioncurrent occurring at the free electrode potential of the measuredcorrosion interface is indicated from a consideration of the basis forsimplified method operation described earlier in this specification, andleads to the following generalizations.

(1) The accuracy of measuurement made on the electronic conductor 1interface from a single value of polarizing DC. current and resultingpolarization voltage is increased by a second measurement made withreversed polarity of the applied DC. voltage.

(2) As the measured value of the resulting polarization voltage producedon the electronic conductor 1 interface is increased beyond the voltage(l/2)(direct voltage), the value of the polarizing DC. current tends toproduce increasing distortion of corrosion interface propertiesoccurring before starting the measurement, and thereby to reduce theaccuracy of the corrosion current measurement.

(3) Measurement made with the resulting polarization voltage produced onthe electronic conductor 1 in terface within a range from slightly aboveto about 0.01 volt may tend to improve accuracy through measurement madewithin the linear range of initial current-potential relationshipextending from free electrode potential E; up to the potential of firsttransition point 17 or 18 of FIGURE 2, and through a minimizing ofcurrent-potential relationship distortion produced by value ofpolarizing DC. current, but may also tend to reduce the precision ofmeasuring the smaller values of polarizing DC. current and resultingpolarization voltage.

(4) When the free electrode potential E; and the resulting polarizedelectrode potential are measured between electronic conductors 1 and 3,correction may be made for IR voltage loss in ionic conduction of themeasured value of the polarizing DC. current by subtracting a smallvoltage from the polarization voltage calculated from the two electrodepotential measurements. This subtraction is more convenientlyaccomplished in practice by increasing the ratio of the direct voltageterm to the resulting polarization voltage through the addition of avoltage up to about 0.003 volt to the direct voltage.

(5) Measurement made with the electronic conductor 1 interface of area Aand the electronic conductor 3 interface of area A electrochemicallyduplicating the A interface and of area equal to the A interface,increases accuracy through measurement of a corrosion current whichaverages the performances of two duplicated interfaces. A secondmeasurement immediately following the first measurement and made withreversed polarity of the applied DC. voltage increases accuracy throughaveraging two corrosion current measurements and through averaging theresponse of each of the tWo corrosion interfaces to polarizing DC.current passed in cathodic direction and in anodic direction.

The degree of accuracy with which the simplified method is capable ofmeasuring the interface electrode system corrosion current may not beindicated through comparison with the corrosion current of the methodmeasuring initial range of current-potential relationship described inthe preceding division of this invention. Measurement by the simplifiedmethod may be made with minimum disturbance of corrosion interfaceproperties occurring before measurement. Measurement of initial range ofcurrent-potential relationship may slightly disturb interface propertiesoccurring before the measurement. A difference in value of corrosioncurrent measured by these two methods may indicate the extent ofdisturbance produced by measurement of initial current-potentialrelationship range.

The accuracy with which the interface electrode system corrosion currentmeasured by the simplified method determines the corrosion rate cannotbe directly measured, since no method other than that of the precedingdivision of this invention is known for measuring corrosion ratenon-destructively and instantaneously. The determination must be madeindirectly through comparison with a corrosion quantity-time measurementmethod, such as the weight loss method, through the following steps.

(1) Measurement is made of the initial Weight of electronic-conductor 1.When corrosion interface compoi2 sition and operation include detailswhich minimize difference in weight loss obtained with duplicatedcorrosion interfaces, opposed electrode 3 may duplicate electrochemically and in area the electronic conductor 1 interface and also bemeasured.

(2) After forming the electronic conductor 1 and 3 interfaces, a seriesof corrosion current measurements are made by the method of thisinvention throughout the progress of the corrosion. A corrosioncurrent-time graph is made with a curve drawn through each measuredpoint of corrosion current and time. The trend of this curve aids indefining the frequency and number of corrosion current measurementsrequired.

(3) The corrosion current-time curve is integrated to a corrosionquantity-time curve through application of Faradays law of electrolysis,with anodic electrochemical reaction assumed to produce metal ions ofvalence equal to that found in the corrosion product. This corrosionquantity-time curve continuously predicts Weighable metal loss.

(4) The corrosion is continued through a duration producing metal lossweighable within about i5% precision, since this is generally the limitof precision within which metal loss occurs on duplicated electrodes.

(5) When metal loss of selected quantity is measured by the corrosionquantity-time curve, the electronic conductors are removed from thecorrosive and are immediately cleaned, dried, and subsequently weighed.The series of corrosion current measurements made at spaced timeintervals by the method of this invention when operated throughduplicated interfaces A and A of equal area, generally measures quantityof metal loss of value between the two quantities of metal loss weighedon the duplicated electrodes. When the weighed quantity of metal loss onduplicated corrosion interfaces is not in close agreement, a corrosionquantity-time curve may be measured separately on each electronicconductor interface by the method of this invention.

With certain interface compositions and conditions of operation, it issometimes found that the valence of metal ions produced in the anodicelectrochemical reaction by the corrosion current of the interfaceelectrode system is different from the valence of the metal ions foundin the corrosion product, particularly in the presence of dissolvedoxygen. When this occurs, it is found that the corrosion quantity-timecurve may be related to the corrosion current-time curve through asimple and exact mathematical expression, which adds classificationdetail to anodic corrosion mechanism. This is taken as evidence thatpurely chemical corrosion reaction may sometimes follow after theinitiating anodic electrochemical reaction to modify the corrosionproducts of the electrochemical reaction.

The non-destructive instantaneous corrosion current measurement by themethod of this invention permits measurement of the effect of acorrosion variable operating to change the corrosion rate of thecorrosion interface While the value of the corrosion variable is changedin a selected manner through a selected range. The corrosion variablemay pertain to ionic conductor compostion, as inhibitor concentration,pH value, salt concentration, or other variables. The corrosion variablemay pertain to corrosive environment, as temperature, flow rate, orother variables. Detailed information is thus obtained with a minimum oftime and effort before weighable quantity of metal loss may occur.

The preceding portion of the specification describes the basis foroperation of the simplified method, the method details of measuringvalue of the polarizing DC. current and resulting polarization voltage,and alternative interface combinations through which the method may beapplied. The principal purpose of the examples which follow is to citespecific evidence of the scope and accuracy of the corrosion ratemeasurement.

Example I.Separate Reference Electrode, Continuous Voltage Delivery Themain purpose of this example is to illustrate measurement of thecorrosion current made with a separate reference electrode 4, with theDC. voltage delivered continuously at a small constant rate of change,and to illustrate the accuracy of measurement indicated throughcorrelation with weighed metal loss.

Electronic conductor 1 was steel sheet in the form of a strip 1.0centimeter wide and 2.5 centimeter long, with both faces and three edgesexposed. Electronic conductor 3 was a duplication of electronicconductor 1. Electrode surfaces were polished with #300 emery paper, andeach electronic conductor was weighed. An electrical lead wire wassoldered to a tab portion of each electronic conductor, and the wire andtab were mounted in a glass tube and sealed with wax. Major axes of theelectrodes were vertically positioned at 1 inch separation with faces ofthe electrodes in a common plane, permissible with the high conductivityof the ionic conductor.

The ionic conductor was l-normal sulphuric acid made up with distilledwater. It was initially deaerated by heating to boiling temperatureunder a layer of white mineral oil, then cooled to room temperature andpartially protected from atmospheric oxygen during the corrosion by a Ainch layer of the oil. The electrode surfaces were lightly scrubbed withwet pumice powder to produce surfaces free from water-break, and thenlowered into the ionic conductor through the oil layer.

The film of water on the electrode surfaces avoided adhesion of the oil.

The corrosive environment included complete submersion of electrodesurfaces, no flow of ionic conductor, maintained deaeration, andtemperature of 22 C.

A saturated calomel reference electrode 4 of noncontaminating junctionwas positioned about A inch from electrode 1, and located so thatelectrode 1 was between electrodes 4 and 3. Free electrode potential E:was measured just before applying the DC. voltage.

The voltage delivery system consisted of a potentiometer andcenter-tapped resistor each connected across the battery. The arm of thepotentiometer was driven at constant speed. The voltage applied to theelectronic conductors was taken from between the center-tap of theresistor and the arm of the potentiometer.

This voltage delivery system was adjusted to initially apply DC. voltageto electronic conductors 1 and 3 of more than 0.02 volt. The voltage wasthen delivered at the substantially constant rate of change of about0.007 volt per minute which decreased the applied DC. voltage to zeroand then increased it in reversed polarity. Measurement was made of thepolarizing DC. current i and the resulting polarized electrode potentiale at an instant of time during the cathodic polarization of theelectronic conductor 1 interface and also at an instant of time duringthe anodic polarization. These measurements were made at 6.0, 35.5, and59.0 hours after starting the corrosion. The corrosion was terminated at60.0 hours. Throughout this duration the electrodes remained inundisturbed contact with the ionic conductor.

The data of the measurements is summarized in Table I which follows. Thepolarization voltage e was calculated as the positive voltage differentbetween potentials e and E The corrosion current i Was then related tothe polarizing DC. current i in the ratio that /1) (direct voltage),taken as 0.016 volt, was related to the polarization voltage e Thevalues of i obtained with cathodic polarization and with anodicpolarization are shown in Table I, with the average value of icalculated from them.

TABLE I.DATA SUMMARIZING CORROSION CURRENT MEASUREMENT OF A SINGLECORROSION INTERFACE The avenage value of the corrosion current measuredat each of these three time intervals spaced throughout the duration ofthe corrosion was graphed to a linear current axis and a linear timeaxis. A smooth curve was drawn through the three points and indicatedrapid increase of corrosion rate during the first 40 hours, with aconstant rate being approached near 60 hours. The corrosion current wasregarded to operate through the anodic electrochemical reaction, Fe=Fe++2(--), so that application of Faradays law of electrolysis producedcalculation of the factor, 0.0104 milligram loss of iron per microampereper ten hours of corrosion. Integration of the corrosion current-timecurve with this calculated factor produced a graphed corrosionquantity-time relationship of increasing slope, which prepicted a 25.8milligram iron loss at 60.0 hours.

The corrosion was terminated at 60.0 hours by removal of the electronicconductors, and their surfaces were immediately rinsed and dried. Theelectrodes were removed from the electrode assemblies, the solderedconnection was removed wtih the solder, and each electrode Was weighed.Weighted metal losses were and 24.0:05 milligram. The predicted metalloss of 25.8 mg. is 8% above the 24.0 mg. loss measured by weighing.

Example 2.-Duplicated Interfaces A and A of Equal Area The main purposeof this example is to illustrate detailed measurement of the corrosioncurrent-time relationship by a plurality of corrosion currentmeasurements spaced at intervals throughout the total duration of thecorrosion, and the accuracy obtainable with duplicated interfaces A andA of equal area.

Measurement was made of the same electronic conductors, ionic conductor,and environment described in Example 1. Electronic conductor 1 formedthe measured interface of area A and electronic conductor 3 formed theopposed interface of area A duplicating A in electrochemical propertiesand physical area, and operating as the reference electrode with the 1inch separation between the major axis of the electronic conductorsintroducing negligible IR voltage loss with the high conductivity of theionic conductor. Electrode 4 was not required. Voltmeter 9 was connectedacross electronic conductors 1 and 3 by switch 11..

Free electrode potential E was measured between electronic conductors 1and 3, and found to be negligible, or substantially equal to zero withthe deaerated ionic conductor. Polarized electrode potential emeasurable between electronic conductors 1 and 3, then equaled thepolarization voltage e of anodic and cathodic polarizations produced onthe duplicated interfaces by the same polarizing DC. current i Atinstants of time selected throughout the total duration of thecorrosion, measurement of corrosion current i was made as follows. TheDC. voltage applied to electronic conductors 1 and 3 was deliveredthrough a low resistance potentiometer. The voltage delivery system wasadjusted to polarize the measured interface at the time of measurementwithin the range of total polarization voltage e extending up to (0.03)(l+A /A :006

volt. Measurement was made of polarizing current i and resultingpolarization voltage e upon the initial attainment of approachedequilibrium in the form of substantially no change in current i firstoccurring through a 20 second time interval. This required a time lapseof about 2 minutes after the DC. voltage application. The applied DC.voltage was then reversed by switch 16, and measurement #2 was made ofvoltage 2;, and current i Since e was maintained constant with the #1and #2 measurements, an averaged i value was calculated from the twomeasured values. The corrosion current i was then related to thepolarizing D.C. current i in the ratio that the term (1/2)(1+A /A(direct voitage) was related to the polarization voltage e The directvoltage was taken as 0.031 volt. The measurements are summarized inTable II which follows.

TABLE II.DATA SUMMARIZING CORROSION CURRENT MEASUREMENT OF DUPLIOATEDINTERFACES F EQUAL AREA Measure- Time of i Microampcres Corrosion ment;Measure e Current, Number ment, Volt 1' m Eours #1 #2 Average The efiectof regarding Free Electrode Potential E as negligible is shown in TableII by relatively large differences between the #1 and #2 values ofpolarizing DC. current i measured during the first three hours of thecorrosion. It can be shown that this effect of small E; voltage iscancelled out by averaging the #1 and #2 measurements of current iobtained with reversal of applied DC. voltage polarity.

The data of Table H was graphed to a linear current axis and a lineartime axis. Measurement number one, in which the #1 value of current iwas measured during the first three to five minutes after immersing theelectronic conductors in the ionic conductor, illustrates the ability tomeasure corrosion current as soon as the corrosion interface is formed.The closely spaced measurements made during the first four hours defineregular curvature, and illustrate the detail with which measurement maybe made of initial changes occurring to the metal surface and to thecorrosive layer contacting it. These simplified method measurements werealso directed to measuring the corrosion current operating before andafter each of three corrosion current measurements made by measuringrange of current-potential relationship defining the first twotransition points on each side of the free electrode potential, asdescribed in my prior method. Each of these current-potentialrelationship range measurements required a time lapse of about 14minutes. At six hours, the transition points measured a 220 microamperecorrosion current, and the simplified method measurements in Table IIshow that the etfect was to increase the corrosion current from 206microamperes before the measurement to 277 microamperes after themeasurement. At 35 .5 hours, the transition points measured a 410microampere current, and Table II shows that the corrosion current wasincreased from 362 to 413 microarnperes. At 59.0 hours, the transitionpoints measured a 550 microampere corrosion current, and Table II showsthat corrosion current was increased from 538 to 574 microamperes. Ingraphing the Table II data, measurements made at 7.0 and 34.8 hours wereconnected by a smooth curve, and measurements made at 36.0 and 59.5hours were connected by a smooth curve.

The graphed corrosion current-time relationship was integrated to acorrosion quantity-time relationship which predicted a metal loss of23.9 milligrams at 60.0 hours. This falls between the values of 230105and 24010.5 milligrams obtained by weighings made after termination ofthe corrosion at 60.0 hours.

Example 3.-Substantially Unpolarized Interface A It is generally knownthat certain combinations of corrosive composition and corrosiveenvironment may produce a condition of unstable corrosion resistanceupon a metal surface, with the result that weight losses measured onduplicated surfaces are not in close agreement. One purpose of thisexample is to illustrate that the method of this invention accuratelymeasures the different metal loss which occurs to each of two duplicatedelectrodes. Another purpose is to illustrate measurement made withelectronic conductor 1 opposed by electronic conductor 3 in the form ofthe inside surface of a cylinder.

Electronic conductor 1 was duplicated in the form of electrodes A and B,each of pure tin sheet of 0.8 cm. thickness, 6.0 cm. length, and 1.0 cm.width. The exposed surface of each electrode comprised both faces andthree edges. The initial weight of each was measured. An electrical leadwire was attached to a tab portion of each electrode and insulated fromthe ionic conductor. Opposed electronic conductor 3 was in the form ofthe inside surface of a pure tin foil cylinder of 4.0 cm. diameter and10 cm. length, with a tab portion for making electrical connection abovethe ionic conductor.

The corrosive consisted of 900 cc. distilled water, 9.0 g. purepotassium chloride, and 2.0 cc. pure acetic acid. It was contained in aglass beaker as a volume of 12 cm. depth, and was exposed to theatmosphere at room temperature of 70 F.

The tin foil cylinder was mounted in the corrosive with the major axisof the cylinder in a vertical position and with the ends of the cylinderequally spaced from the bottom and top of the corrosive. The twoduplicated electrodes were mounted inside the cylinder, with their majoraxes in 'a vertical position and separated by 1.5 cm. spacing, with theends of the electrodes equally spaced from the ends of the cylinder.

Corrosion current was measured by the simplified method at a selectedinstant of time during the progress of the corrosion as follows. Thefree electrode potential of electrode A was measured as Zero from thecylinder electrode by connecting one terminal of a resistor of about oneohm to one of these two electrodes, selecting a polarity of equalizingDC. voltage to be applied across the resistor in opposition to thepolarity of the unequalized free electrode potential difference betweenthe two electrodes, and applying this equalizing DC. voltage in valueadjusted to produce zero voltage difference between the other terminalof the resistor and the other electrode. The D.C. voltage of thesimplified method was then applied to this other resistor terminal andthis other electrode through a voltage delivery system maintaining theapplied DC. voltage constant. Measurement was made of the polarizingD.C. current i and the resulting polarization voltage e in the form ofthe applied DC. voltage, When substantially no change in DC. current ifirst occurred over a 20 second interval after DC. voltage application.The D.C. voltage of the simplified method was then applied in reversedpolarity and a #2 measurement was made of polarizing DC. current i Theequalizing DC. voltage and the measuring DC. voltage were then removed.In the same manner, #1 and #2 measurements were then made betweenelectrode B and the cylinder electrode. The applied DC. voltage, equalto resulting polari 1 7 zation voltage e was 0.01445 volt. The arearatio was calculated from the electrode dimensions as,

A voltage of 0.003 volt was added to the 0.032 volt value of the directvoltage to correct for IR voltage loss in the ionic conduction of thepolarizing DC. current, i The corrosion current i was then related tothe polarizing DC current i,,, in the ratio that the term (1/2) (l+A /A(direct voltage) RENT MEASUREMENT \VITH SUBSTANTIALLY UN- POLARIZEDOPPOSED INTERFACE Corrosion current im, microamperes Time of Measuremeasuremont ment, A and cylinder B and cylinder I number hours #1 #2Average #1 #2 Average This data was graphed to produce two corrosioncurrenttime relationships. These were integrated to two corrosionquantity-time relationships according to the anodic reaction, Sn=Sn ++2(through the factor, 0.021 milligram per microampere per 10 hourscalculated from Faradays law of electrolysis, and predicted metal lossesat 73 hours of corrosion as A=20.6 mg., and B=14.9 mg.

At 73.0 hours the corrosion was terminated by removal of electrodes Aand B from the corrosive, followed by the rinsing, wiping, and drying oftheir surfaces. The surfaces showed scattered severe pitting, withetched surface of about 90% and unetched surface of about 10%, furtherconfirming corrosion system instability. Measured weight losses were,A=23.0i:0.5 mg., and B=13.0i0.5 mg.

The simplified method measurement made on electrode A differed from theweighed metal loss by the percentage (100) (20.523)/(23)=10%, and themeasurement made on electrode B differed from weighed metal loss by thepercentage (100)(l4.9l3)/(l3)=+9%. The graphs made from the simplifiedmethod measurements continuously related corrosion performance withtime. The weighed metal losses showed only what had happened up to thetime when the electrodes were removed from the ionic conductor forweighing.

Example 4.Anodic Reaction Measurement The main purpose of this exampleis to illustrate electrochemical corrosion reaction operating at avalence differing from the valence of the metal in the corrosionproduct.

Duplicated electronic conductors of pure aluminum were corroded by0.5-normal sulphuric acid at room temperature, and the initial trend ofthe corrosion current-time relationship was measured by the simplifiedmethod of this invention. Sodium chloride was then added in quantityproducing 0.1.-N concentration and the trend of the greatly acceleratedcorrosion current-time relationship was measured. In view of thecorrosion products, Al (SO and AlCl the graphed current-timerelationship was integrated to the corrosion quantity-time relationshipaccording to the anodic reaction, Al=Al ++3 and predicted a 16.6milligram metal loss. and 20 milligrams were measured by weighing. Thelow value of the predicted metal loss suggested the possibility that theanodic reaction, Al=Al ++1(), might occur in the chloride-free sulphuricacid, to be followed by dependent chemical reaction producing Al ions.Integration according to this hypothesis predicted an 18.4 milligrammetal loss. Evidence confirming this hypothesis was obtained bycorroding duplicated aluminum electrodes in l-normal sulphuric acidwithout sodium chloride addi tion. Measurement by the method of thisinvention predicted a 22.3 milligram metal loss according to Alformation in anodic electrochemical reaction, which agreed with a23.0105 milligram metal loss obtained by weighing.

ALTERNATIVES The simplified method of this invention is found to produceaccurate corrosion rate measurement throughout wide range of variationmade in electronic conductor composition, ionic conductor composition,physical factors of corrosion environment, and duration of thecorrosion, and confirms the characteristics of the interface electrodesystem described and measured in my prior method. The possibility isnevertheless recognized that an exceptional corrosion interface might beencountered in practice, having characteristic voltage separationbetween consecutive transition points and characteristic line slopevoltage differing from those defining the direct voltage as 0032:0003volt. The method of this invention would remain applicable to such anexceptional corrosion interface through the value of the direct voltagedefined from interface electrode system measurement by the method of myprior method.

The method of this invention is described in the detail which includesthe requirements for obtaining maximum accuracy of corrosion currentmeasurement, but it is not restricted to high accuracy and may beapplied to produce qualitative measurements. The method of thisinvention is regarded to be applied regardless of the accuracy withwhich it is operated.

I claim:

1. The method of measuring a plurality of individual values of thecorrosion current existing at different times within an interface in acorrosion cell to thereby obtain an indication of the differentcorrosion rate existing at each particular time, including the steps ofestablishing Within a non-gaseous ionic conductor two duplicatedelectrodes forming interfaces of substantially equal area, applying aDC. voltage e between the electronic conductors of said interfaces ofvalue selected from within the range from slightly above zero to 0.040volt for each different measurement of the corrosion current at eachdifferent time, continuing the application of said voltage for thatperiod of time required to initially produce the same selected smallrate of change in value of current for each different measurement of thecorrosion current, and measuring the current i at the end of said periodof time for each particular measurement of the corrosion current, andimmediately thereafter discontinuing the voltage for that particularmeasurement, whereby in each particular measurement the corrosioncurrent i at the free electrode potential, becomes measured from therelationship, i =(E /e (i where E is a constant value of voltageselected from within the range of 0.029 to 0.035 volt.

2. The method of measuring current in a corrosion cell indicative ofcorrosion current at the free electrode potential of an electrodeincluding the steps of establishing within a non-gaseous ionic conductortwo duplicated Metal losses of 18 electrodes forming interfaces ofregular shape and contour, applying a direct current voltage selectedwithin the range of slightly above zero to about .040 volt to theelectrodes, measuring the current passed through the interfaces when thecurrent is substantially equal to the current value existing whensubstantially no change in current first occurs over a small timeinterval following application of voltage and before any visible changeappears in the interfaces following application of said voltage,.andmeasuring the voltage at the time of current measurement, whereby thecurrent measured is proportional to the corrosion current by theproportion of the measured voltage to a voltage within the range of .029to .035 volt.

3. The method of measuring current in a corrosion cell indicative ofcorrosion current at the free electrode potential of an electrodeincluding the steps of establishing within a non-gaseous ionic conductortwo duplicated electrodes forming interfaces of regular shape andcontour, applying a direct current voltage selected within the range ofslightly above zero to about .040 volt to the electrodes, measuring thecurrent passed through the interfaces when the current is substantiallyequal to the current value existing when substantially no change incurrent first occurs over a time interval of approximately twenty tothirty seconds following application of voltage and before any visiblechange appears in the interfaces following application of said voltage,and measuring the voltage at the time of the current measurement,whereby the current measured is proportional to the corrosion current bythe proportion of the measured voltage to a voltage within the range of.029 to .035 volt.

4. The method of measuring current in a corrosion cell indicative ofcorrosion current at the free electrode potential of an electrodeincluding the steps of establishing within a non-gaseous ionic conductortwo duplicated electrodes forming interfaces of regular shape andcontour, applying a direct current voltage selected within the range ofslightly above zero to about .040 volt to the electrodes, measuring thecurrent passed through the inter faces when the current is substantiallyequal to the current value existing when'substantially no change incurrent first occurs over a small time interval following application of.voltage and before any visible change appears in the interfacesfollowing application of said voltage, whereby the current measured isproportional to the corrosion current by the proportion of the measuredvoltage to a voltage within'the range of .029 to .035 volt, measuringthe voltage at the time of the current measurement and immediatelythereafter reversing the polarity of the applied voltage and againtaking a current reading under the same condition aforementioned therebyto obtain an average value of corrosion current.

5. The method of measuring current in a corrosion cell indicative ofcorosion current at the free electrode potential of an electrodeincluding the steps of establishing within a non-gaseous ionic conductortwo duplicated electrodes forming interfaces of regular shape andcontour, applying a direct current voltage such as to produce a totalpolarization voltage in the range of from slightly above Zero to about.040 volt at the time of subsequent measurement to the electrodes, andmeasuring the current passed through the interfaces and totalpolarization volt age when substantially no change in thecurrent-potential relationship first occurs following application of thevoltage and before any visible change appears in the interfacesfollowing application of said voltage, whereby the current measured isproportional to the corrosion current by the proportion of the measuredvoltage to a vol age Within the range of .029 to .035 volt.

6. The method of measuring current in a corrosion cell indicative ofcorrosion current at the free electrode potential of an electrodeincluding the steps of establishing within a non-gaseous ionic conductortwo electrodes forming duplicated interfaces of regular shape andcontour and of areas A and A where A is not less than A applying adirect current voltage selected within the range of slightly above zeroto about 1/2(.04) (1+A /A volts to the electrodes, measuring the currentpassed through the interfaces when the current is substantially equal tothe current value existing when substantially no change in current firstoccurs over a small time interval following application of voltage andbefore any visible change appears in the interfaces followingapplication of said voltage, and measuring the voltage at the time ofcurrent measurement, whereby the current measured is proportional to thecorrosion current by the proportion of the measured voltage to a voltagewithin the range of 1/2(.029) (1+A /A to 1/2(.035) (1+A /A volts.

References Cited in the file of this patent Blum et al.: Transaction ofthe American Electrochemical Society, vol. 52, 1927, pages 403429.

UNITED STATES PATENT OFFICE CERTIFICATE OF GORRE-CTIN Patent No 3 069332 December 18 1962 Robert GO Seyl It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 1 line 21 after 'doned o insert the following as a new paragraphsThe present application deals with a simplification of a methoddescribed and claimed in my co--pending application Serial Non 840 266filed on September 16 1959 which method of that appli cation ishereinafter referred to as my prior methoda same column 1 line 69 0beginning with "New terminologies strike out all to and includingdescribed and meas in line 72 same column .1; column .2 line 72 afterpolara insert we bilities of these interface electrodes described andmeas column 9 line 29 after of insert a "6 Signed and sealed this 26thday of May 196410 (SEAL) Attest:

ERNEST WC SWIDER EDWARD JO BRENNER Attesting Officer Commissioner ofPatents

1. THE METHOD OF MEASURING A PLURALITY OF INDIVIDUAL VALUES OF THECORROSION CURRENT EXISTING AT DIFFERENT TIMES WITHIN AN INTERRFACE IN ACORROSION CELL TO THEREBY OBTAIN AN INDICATION OF THE DIFFERENTCORROSION RATE EXISTING AT EACH PARTICULAR TIME, INCLUDING THE STEPS OFESTABLISHING WITHIN A NON-GASEOUS IONIC CONDUCTOR TWO DUPLICATEDELECTRODES FORMING INTERFACES OF SUBSTANTIALLY EQUAL AREA, APPLYING AD.C. VOLTAGE EP, BETWEEN THE ELECTRONIC CONDUCTORS OF SAID INTERFACES OFVALUE SELECTED FROM WITHIN THE RANGE FROM SLIGHTLY ABOVE ZERO TO 0.040VOLT FOR EACH DIFFERENT MEASUREMENT OF THE CORROSION CURRENT AT EACHDIFFERENT TIME, CONTINUING THE APPLICATION OF SAID VOLTAGE FOR THATPERIOD OF TIME REQUIRED TO INITIALLY PRODUCE THE SAME SELECTED SMALLRATE OF CHANGE IN VALUE OF CURRENT FOR EACH DIFFERENT MEASUREMENT OF THECORROSION CURRENT, AND MEASURING THE CURRENT IP AT THE END OF SAIDPERIOD OF TIME FOR EACH PARTICULAR MEASUREMENT OF THE CORROSION CURRENT,AND IMMEDIATELY THEREAFTER DISCONTINUING THE VOLTAGE FOR THAT PARTICULARMEASUREMENT, WHEREBY IN EACH PARTICUL MEASUREMENT THE CORROSION CURRENTIC AT THE FREE ELECTRODE POTENTIAL, BECOMES MEASURED FROM THERELATIONSHIP, IC=(ED/EP) (IP), WHERE ED IS A CONSTANT VALUE OF VOLTAGESELECTED FROM WITHIN THE RANGE OF 0.029 TO 0.035 VOLT.